1. Field of the Invention
This invention is directed to solid electrolytes containing additives including a toughening agent and/or a basic sink material and, in particular, to solid electrolytes containing a polymeric matrix, a salt, a solvent, and said additives. The toughening agent imparts mechanical strength to the solid electrolyte and the basic sink removes or traps acids (e.g., HF) in the solid electrolyte.
2. State of the Art
Electrolytic cells containing an anode, a cathode and a solid, solvent-containing electrolyte incorporating a salt are known in the art and are usually referred to as xe2x80x9csolid batteriesxe2x80x9d. See, for instance, U.S. Pat. Nos. 5,229,225, 5,238,758, 5,358,801, and 5,366,928. These cells offer a number of advantages over electrolytic cells containing a liquid electrolyte (i.e., xe2x80x9cliquid batteriesxe2x80x9d) including improved safety features.
Solid batteries employ a solid electrolyte interposed between a cathode and an anode. The solid electrolyte contains either an inorganic or an organic matrix and a suitable salt, such as an inorganic ion salt, as a separate component. Electrolytic cells containing a solid electrolyte having a polymeric matrix suffer from low ion conductivity and, accordingly, in order to maximize the conductivity of these materials, the matrix is generally constructed as a very thin film, i.e., in the range of about 25 to about 250 xcexcm. Minimizing the thickness of the film reduces the total amount of internal resistance within the electrolyte but also decreases the solid electrolyte""s structural integrity. In addition, good adherence of the anode and cathode to the solid electrolyte is necessary for optimum operation of electrochemical cells made therefrom.
Another problem encountered in electrolytic cells is the presence of impurities such as acids (e.g., HF) in the solid and liquid electrolytes. HF is derived from certain lithium salts (e.g., LiPF6) that are employed. For example, LiPF6 reacts with water to form HF, LiF (insoluble) and other by-products, thereby reducing the amount of salt available. The acids adversely effect electrochemical performance.
In view of the above, the art is in need of solid electrolytes having superior mechanical attributes, including toughness, hardness, and resiliency. In addition, the solid electrolyte should also adhere to the anode and cathode layers of the electrolytic cell to minimize internal resistance and increase electrochemical performance. Furthermore, there is a need to reduce or eliminate acidic impurities in the solid electrolyte.
The present invention is based, in part, to the discovery that adding a toughening agent to an electrolyte composition yields a stronger solid electrolyte that adheres well to the anode and cathode. Suitable toughening agents comprise alumina, silica, zeolites, metal oxides and mixtures thereof. The invention is also based in part on the discovery that alumina (Al2O3) acts as a base sink to remove or trap acidic impurities, especially HF.
In one aspect, the present invention is directed to an electrolytic cell which comprises: an anode; a cathode and a solid, solvent-containing electrolyte which comprises a polymeric matrix, a salt, a solvent, and a toughening agent, wherein the electrolyte is interposed between the anode and cathode.
In another aspect, the present invention is directed to a process for preparing the electrolytic cells which demonstrate improved electrochemical performance.
This invention is directed to a solid, solvent-containing electrolyte comprising a toughening agent. One aspect of the invention is that the presence of the toughening agent enhances the mechanical strength of the solid electrolyte. Another aspect is that the solid electrolyte layer also demonstrates good adherence to the cathode and/or anode layers. It is believed that improvement in the structural integrity of the solid electrolyte is due, in part, to the ability of the toughening agent to also function as an absorbent (i.e., drying agent) to remove water, excess solvents and impurities. In particular, alumina, a toughening agent, also functions as a basic sink to remove a significant part of the acids from the solid electrolyte which improves cell performance and cycle life.
However, prior to describing this invention in further detail, the following terms are defined as follows:
The term xe2x80x9csolid polymeric matrixxe2x80x9d refers to an electrolyte compatible material formed by polymerizing organic monomers (or partial polymers thereof) and which, when used in combination with the other components of the electrolyte, renders the electrolyte solid.
The term xe2x80x9ca solid matrix forming monomerxe2x80x9d refers to organic materials which in monomeric form can be polymerized, preferably in the presence of an inorganic ion salt, and a solvent to form solid matrices which are suitable for use as solid electrolytes in electrolytic cells. Examples of suitable organic solid matrix forming monomers include, by way of example, propylene oxide, ethyleneimine, ethylene oxide, epichlorohydrin, acryloyl-derivatized polyalkylene oxides (as disclosed in U.S. Pat. No. 4,908,283), urethane acrylate, vinyl sulfonate polyalkylene oxides (as disclosed in U.S. Pat. No. 5,262,253, which patent is incorporated herein by reference in its entirety), and the like as well as mixtures thereof.
The term xe2x80x9ca partial polymer of a solid matrix forming monomerxe2x80x9d refers to solid matrix forming monomers which have been partially polymerized to form reactive oligomers. Partial polymerization may be conducted for the purpose of enhancing the viscosity of the monomer, decreasing the volatility of the monomer, and the like. Partial polymerization is generally permitted so long as the resulting partial polymer can be further polymerized, preferably in the presence of a solvent, such as, a mixture of organic carbonate(s) to form solid polymeric matrices which are suitable for use as solid electrolytes in electrolytic cells.
The term xe2x80x9ccuredxe2x80x9d or xe2x80x9ccured productxe2x80x9d refers to the treatment of the solid matrix forming monomer or partial polymer thereof under polymerization conditions (including cross-linking) so as to form a solid polymeric matrix. Suitable polymerization conditions are well known in the art and include by way of example, heating the monomer, irradiating the monomer with UV light, electron beams, and the like. Examples of suitable cured products suitable for use in this invention are set forth in U.S. Pat. Nos. 4,830,939 and 4,990,413 which are incorporated herein by reference in their entirety.
The solid matrix forming monomer or partial polymer can be cured or further cured prior to or after addition of the salt, solvent, and toughening agent, and, optionally, a viscosifier. For example, a composition comprising requisite amounts of the solid matrix forming monomer, salt, organic carbonate solvent, viscosifier and toughening agent can be applied to a substrate and then cured. Alternatively, the matrix forming monomer can be first cured and then dissolved in a suitable volatile solvent. Requisite amounts of the salt, organic carbonate solvent, viscosifier and toughening agent can then be added. The mixture is then placed on a substrate; removal of the volatile solvent would result in the formation of a solid electrolyte. In either case, the resulting solid electrolyte would be a homogeneous, single phase product which is maintained upon curing, and does not readily separate upon cooling to temperatures below room temperature. Accordingly, the solid electrolyte of this invention does not require a separator as is typical of liquid electrolytes.
Alternatively, the solid polymeric matrix can be formed by a casting process which does not require the use of monomers or prepolymers, that is, no curing is required. A preferred method employs a copolymer of polyvinylidenedifluroide and hexafluoropropylene dissolved in acetone or other suitable solvent(s). Upon casting the solution, the solvent is evaporated to form the solid polymeric matrix. The solution may be casted directly onto a current collector. Alternatively, the solution is casted onto a substrate, such as a carrier web, and after the solvent (e.g., acetone) is removed, an electrode film is formed thereon. Preferably, the toughening agent and/or basic sink material is incorporated into the solid polymeric matrix by adding the same to the solution prior to casting. Casting techniques of preparing electrolytic cells are described for in U.S. patent application Ser. No. 08/559,121 filed on Nov. 17, 1995, which application is incorporated herein.
The term xe2x80x9csaltxe2x80x9d refers to any salt, for example, an inorganic salt, which is suitable for use in a solid electrolyte. Representative examples of suitable inorganic ion salts are alkali metal salts of less mobile anions of weak bases having a large anionic radius. Examples of such anions are Ixe2x88x92, Brxe2x88x92, SCNxe2x88x92, ClO4xe2x88x92, BF4xe2x88x92, PF6xe2x88x92, AsF6xe2x88x92, CF3COOxe2x88x92, CF3SO3xe2x88x92, N(SO2CF3)2xe2x88x92, and the like. Specific examples of suitable inorganic ion salts include LiClO4, LiSCN, LiBF4, LiAsF6, LiCF3SO3, LiPF6, (CF3SO2)2NLi, (CF3SO2)3CLi, NaSCN, and the like. The inorganic ion salt preferably contains at least one cation selected from the group consisting of Li, Na, Cs, Rb, Ag, Cu, Mg and K. When employing salts such as, for example, LiBF4, LiPF6, or LiAsF6, which forms HF in the liquid and solid electrolytes, preferably a base sink material such as alumina is also added to the solid electrolyte to neutralize the HF.
The term xe2x80x9ccompatible electrolyte solventxe2x80x9d or xe2x80x9celectrolytic solvent,xe2x80x9d or in the context of components of the solid electrolyte, just xe2x80x9csolvent,xe2x80x9d is a low molecular weight organic solvent added to the electrolyte and/or the cathode composition, which may also serve the purpose of solvating the inorganic ion salt. The solvent is any compatible, relatively non-volatile, aprotic, relatively polar, solvent. Preferably, these materials have boiling points greater than about 85xc2x0 C. to simplify manufacture and increase the shelf life of the electrolyte/battery. Typical examples of solvent are mixtures of such materials as propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, gamma-butyrolactone, triglyme, tetraglyme, dimethylsulfoxide, dioxolane, sulfolane, and the like. A particularly preferred solvent is disclosed in U.S. Pat. No. 5,262,253, which is incorporated herein solvent is disclosed in U.S. Pat. No. 5,262,253, which is incorporated herein.
For electrochemical cells where the cathode comprises lithiated cobalt oxides, lithiated manganese oxides, or lithiated nickel oxides, and where the anode comprises carbon, the electrolytic solvent preferably comprises a mixture of ethylene carbonate and dimethyl carbonate. For electrochemical cells where the cathode comprises vanadium oxides, e.g., V6O13 and the anode is lithium, the electrolytic solvent preferably comprises a mixture of propylene carbonate and triglyme.
The term xe2x80x9corganic carbonatexe2x80x9d refers to hydrocarbyl carbonate compounds of no more than about 12 carbon atoms and which do not contain any hydroxyl groups. Preferably, the organic carbonate is a linear aliphatic carbonate and most preferably a cyclic aliphatic carbonate.
Suitable cyclic aliphatic carbonates for use in this invention include 1,3-dioxolan-2-one (ethylene carbonate); 4-methyl-1,3-dioxolan-2-one (propylene carbonate); 4,5-dimethyl-1,3-dioxolan-2-one; 4-ethyl-1,3-dioxolan-2-one; 4,4-dimethyl-1,3-dioxolan-2-one; 4-methyl-5-ethyl-1,3-dioxolan-2-one; 4,5-diethyl-1,3-dioxolan-2-one; 4,4diethyl-1,3-dioxolan-2-one; 1,3-dioxan-2-one; 4,4-dimethyl-1,3-dioxan-2-one; 5,5-dimethy-1-1,3-dioxan-2-one; 5-methyl-1,3-dioxan-2-one; 4-methyl-1,3-dioxan-2-one; 5,5-diethyl-1,3-dioxan-2-one; 4,6-dimethyl-1,3-dioxan-2-one; 4,4,6-trimethyl-1,3-dioxan-2-one; and spiro (1,3-oxa-2-cyclohexanone-5xe2x80x2, 5xe2x80x2, 1xe2x80x2, 3xe2x80x2-oxa-2xe2x80x2-cyclohexanone).
Several of these cyclic aliphatic carbonates are commercially available such as propylene carbonate and ethylene carbonate. Alternatively, the cyclic aliphatic carbonates can be readily prepared by well known reactions. For example, reaction of phosgene with a suitable alkane-xcex1,xcex2-diol (dihydroxy alkanes having hydroxyl substituents on adjacent carbon atoms) or an alkane-xcex1,xcex3-diol (dihydroxy alkanes having hydroxyl substituents on carbon atoms in a 1,3 relationship) yields an a cyclic aliphatic carbonate for use within the scope of this invention. See, for instance, U.S. Pat. No. 4,115,206, which is incorporated herein by reference in its entirety.
Likewise, the cyclic aliphatic carbonates useful for this invention may be prepared by transesterification of a suitable alkane-xcex1,xcex2-diol or an alkane-xcex1,xcex3-diol with, e.g., diethyl carbonate under transesterification conditions. See, for instance, U.S. Pat. Nos. 4,384,115 and 4,423,205 which are incorporated herein by reference in their entirety. Additional suitable cyclic aliphatic carbonates are disclosed in U.S. Pat. No. 4,747,850 which is also incorporated herein by reference in its entirety.
The term xe2x80x9cviscosifierxe2x80x9d refers to a suitable viscosifier for solid electrolytes. Viscosifiers include conventional viscosifiers such as those known to one of ordinary skill in the art. Suitable viscosifiers include film forming agents well known in the art which include, by way of example, polyethylene oxide, polypropylene oxide, copolymers thereof, and the like, having a number average molecular weight of at least about 100,000, polyvinylpyrrolidone, carboxymethylcellulose, and the like.
The term xe2x80x9ctoughening agentxe2x80x9d refers to a solid particulate, compatible in solid electrolytes, that enhances the solid electrolyte""s structural integrity and adherence to the anode and cathode surfaces. Suitable solid particulates include alumina (Al2O3), silica (SiO2) and zeolites. Aluminum oxide is most preferred since it can also functions as a basic sink material that removes or neutralizes acids in the electrolyte. The basic sink material can be introduced in either the neutral or basic form because the surface of Al2O3 can be altered. Most likely, it is believed that the surface has the structure: Al(O)(OH) with the number of OH groups determining its basicity. In the neutral or basic form, the OH group on the alumina will strongly attract the F of HF via a hydrogen bond: Al(O)OHxe2x80x94FH. Thus, HF can be viewed as a sink because the HF reacts with or is xe2x80x9cbondedxe2x80x9d to the alumina and will not be released. The hydrogen bond between H and F is one of the strongest. The alumina effectively removes the HF from the system.
Zeolites suitable for use in the present invention include natural zeolites, such as hydrated silicates of aluminum and at least one of sodium and calcium, and synthetic zeolites, such as those made, for example, by a gel process (sodium silicate and alumina) and a clay process (kaolin), which form a matrix to which the zeolite is added and involve the substitution of various rare-earth oxides. Suitable toughening agents also include non-reactive metal oxide materials, e.g., calcium oxide and magnesium oxide. Mixtures of different toughening agents can be used. The toughening agent is preferably in powder form. Preferably, the size of the toughening agent ranges from about 0.1xcexc to about 10xcexc, more preferably from about 0.3xcexc to about 5xcexc, and most preferably from about 1xcexc to about 3xcexc.
Preferably the toughening agent comprises about 0.1% (wt) to about 13% (wt), more preferably about 2% (wt) to about 8% (wt), and most preferably about 3% (wt) to about 6% (wt) of the electrolyte layer.
The term xe2x80x9celectrolytic cellxe2x80x9d or xe2x80x9celectrochemical cellxe2x80x9d refers to a composite containing an anode, a cathode and an ion-conducting electrolyte interposed therebetween.
The term xe2x80x9cbatteryxe2x80x9d refers to two or more electrochemical cells electrically interconnected in an appropriate series/parallel arrangement to provide the required operating voltage and current levels.
The anode is typically comprised of a compatible anodic material which is any material which functions as an anode in a solid electrolytic cell. Such compatible anodic materials are well known in the art and include, by way of example, lithium, lithium alloys, such as alloys of lithium with aluminum, mercury, manganese, iron, zinc, intercalation based anodes such as those employing carbon, tungsten oxides, and the like. Particularly preferred anodes include lithium intercalation anodes employing carbon materials such as graphite, cokes, mesocarbons, and the like. Such carbon intercalation based anodes typically include a polymeric binder suitable for forming a bound porous composite having a molecular weight of from about 1,000 to 5,000,000. Examples of suitable polymeric binders include EPDM (ethylene propylene diene termonomer), PVDF (polyvinylidene difluoride), EAA (ethylene acrylic acid copolymer), EVA (ethylene vinyl acetate copolymer), EAA/EVA copolymers, and the like. The anode also may include an electron conducting material such as carbon black.
The cathode is typically comprised of a compatible cathodic material (i.e., insertion compounds) which is any material which functions as a positive pole in a solid electrolytic cell. Such compatible cathodic materials are well known in the art and include, by way of example, transition metal oxides, sulfides, and selenides, including lithiated compounds thereof. Representative materials include cobalt oxides, manganese oxides, molybdenum oxides, vanadium oxides, sulfides of titanium, molybdenum and niobium, the various chromium oxides, copper oxides, lithiated cobalt oxides, e.g., LiCoO2, lithiated manganese oxides, e.g., LiMn2O4, lithiated nickel oxides, e.g., LiNiO2, and mixtures thereof. Cathode-active material blends of LixMn2O4 (spinel) is described in U.S. Pat. No. 5,429,890 which is incorporated herein. The blends can also include LixMn2O4 (spinel) and at least one lithiated metal oxide selected from LixNiO2 and LixCoO2 wherein 0 less than xxe2x89xa62.
A preferred method of fabricating an electrochemical cell is described herein. In addition, methods for preparing solid electrochemical cells and batteries are described in the art, for example, in U.S. Pat. Nos. 5,300,373, 5,316,556, 5,346,385, 5,262,253, 5,028,500, 4,472,487, and 4,668,595, which are all incorporated herein.
In one preferred embodiment, the cathode is prepared from a cathode paste which comprises from about 35 to 65 weight percent of a compatible cathodic material; from about 1 to 20 weight percent of an electroconductive agent; from about 0 to 20 weight percent of polyethylene oxide having a number average molecular weight of at least 100,000. When the cathode includes V6O13, the cathode paste preferably further comprises from about 10 to 50 weight percent of solvent comprising a 10:1 to 1:4 (w/w) mixture of an organic carbonate and a glyme. The paste further comprises from about 5 weight percent to about 25 weight percent of the solid matrix forming monomer or partial polymer thereof. Also included is an ion conducting amount of an inorganic ion salt. Generally, the amount of the salt is from about 1 to about 25 weight percent. (All weight percents are based on the total weight of the cathode.)
The electrolyte composition typically comprises from about 5 to about 25 weight percent of the inorganic ion salt based on the total weight of the electrolyte; preferably, from about 10 to 20 weight percent; and even more preferably from about 10 to about 15 weight percent. The percentage of salt depends on the type of salt and electrolytic solvent employed.
The electrolyte composition typically comprises from 40 to about 80 weight percent electrolyte solvent (e.g., organic carbonate/glyme mixture) based on the total weight of the electrolyte; preferably from about 60 to about 80 weight percent; and even more preferably about 70 weight percent. The electrolyte composition typically comprises from about 5 to about 30 weight percent of the solid polymeric matrix based on the total weight of the electrolyte; preferably from about 10 to about 20 weight percent. The electrolyte composition comprises from about 1 to about 15 weight percent toughening agent, based on the total weight of the electrolyte; preferably from about 5 to about 10 weight percent.
In a preferred embodiment, the electrolyte composition further comprises a small amount of a film forming agent. Suitable film forming agents are well known in the art and include, by way of example, polyethylene oxide, polypropylene oxide, copolymers thereof, and the like, having a numbered average molecular weight of at least about 100,000. Preferably, the film forming agent is employed in an amount of about 1 to about 10 weight percent and more preferably at about 2.5 weight percent based on the total weight of the electrolyte composition.